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Extragalactic astronomy is the branch of astronomy concerned with objects outside the Milky Way galaxy. In other words, it is the study of all astronomical objects which are not covered by galactic astronomy, the next level of galactic astronomy.
As instrumentation has improved, more distant objects can now be examined in detail. It is therefore useful to sub-divide this branch into Near-Extragalactic Astronomy and Far-Extragalactic Astronomy. The former deals with objects such as the galaxies of the Local Group, which are close enough to allow very detailed analyses of their contents (e.g. supernova remnants, stellar associations). The latter describes the study of objects sufficiently far away that only the brightest phenomena are observable.
Some topics include:
- Galaxy groups
- Galaxy clusters
- Galaxy filaments
- Radio galaxies
- Intergalactic stars
- Intergalactic dust
- Intergalactic dust clouds
- the observable universe
- Andromeda–Milky Way collision
- Galaxy color–magnitude diagram
- Galaxy formation and evolution
- Observational cosmology